Medical and surgical instruments such as needle holders, needle drivers, forceps and occluders that incorporate a box hinge between handle ends and working tip ends are well known. Referring to FIGS. 1 through 3, these instruments include a pair of members joined together by a box hinge. Forming the box hinge one of the members includes a slot having flat, parallel sides and the other member includes a portion machined to conform with the flat, parallel sides of the slot. The machined portion extends through the slot in an assembled instrument. A hinge pin extends through the parallel sides of the slot and through the machined portion of the other member disposed in the slot. It is known that suture thread does snag at or on edges E1 around the machined portion of the one member and around or under machined edges E2 machined around the slot. In an attempt to reduce snags some manufacturers bevel the machined edges, or attempt to provide very expensive and precise matched grinding of the handles at either end of the box hinge where the machined portion first interfaces with the slot.
Even with the beveled edges a suture may snag above and below the pivot point of the hinge as a surgeon is tying a knot in the suture, or when a prior art instrument goes into and out of an array of temporarily placed suture threads placed in, or around, a large wound or opening in tissues; in which one or more layers is held open by multiple instruments and suture threads. Such snagging can occur when preparatory knot loops, or partial turns, are moving either up, or down, the instrument body, and at locations both above, and below, the box hinge. Snagging potential increases directly with smaller suture thread and with larger suture needles (more open jaws and mismatch at the prime snag areas). Likewise, the matched grinding has the disadvantage of being costly typically still snagging when suture thread is of very fine diameter, and still leads to snagging due to mismatch of the interface as the working tip ends are opened slightly to hold the suture needle or other objects.